(a) Field of the Invention
The present invention relates to a manufacturing method of a thin film transistor for liquid crystal displays.
(b) Description of the Related Art
A conventional thin film transistor has a hydrogenated amorphous silicon layer as an active layer. A doped hydrogenated amorphous silicon layer heavily doped with n-type impurities formed on the amorphous silicon layer is used to decrease the contact resistance between the amorphous silicon layer and source/drain electrodes. To form an etch-back type TFT, the doped amorphous silicon layer is etched away using the source/drain electrodes as a mask. However, there is a problem that the source/drain electrodes are consumed in the step of etching the doped amorphous silicon layer.
To prevent-the above-described problem, a photoresist pattern used to form the source/drain electrodes is used as an etching mask in the step of etching the doped amorphous silicon layer and the photoresist pattern is stripped thereafter. However, the amorphous silicon layer experiences a bad influence during the step of stripping the hardened photoresist pattern and the characteristics of the TFT become worse.
In view of the above, an object of the present invention is to improve the characteristics of a TFT for liquid crystal display.
This and other objects, features and advantages are provided, according to the present invention, by performing a He plasma treatment or an oxygen plasma treatment after dry etching the doped amorphous silicon layer used as a ohmic contact layer of a TFT.
The He plasma treatment and the oxygen plasma treatment may be used in case of forming a data pattern including a source and a drain electrodes made of a molybdenum or molybdenum alloy and an aluminum or aluminum alloy, respectively.
After etching the doped amorphous silicon layer, He plasma treatment or the oxygen plasma treatment with in-situ may be performed to improve the characteristics of the TFT or to prevent corrosion the aluminum or aluminum alloys.
When the doped amorphous silicon layer is dry etched, a gas system consisting of a hydrogen halide and at least one selected from CF4, CHF3, CHClF2, CH3F and C2F6 may be used, especially, an HCl+CF4 gas system may be preferable.
Furthermore, HCl+CF4+O2 gas system including an oxygen may be preferable in case of forming a data pattern including a source and a drain electrodes made of a molybdenum or molybdenum alloy to improve the characteristics of the TFT.
The doped amorphous silicon layer may be etched using the source and the drain electrodes or the photoresist pattern used to form the source and the drain electrodes as a mask.